[flink] unify the behavior of different startup mode for lake table

[luoyuxia]

Purpose

Linked issue: close #xxx

Brief change log

Tests

API and Format

Documentation

[Copilot]

Pull request overview

This PR unifies the behavior of different startup modes for lake tables by removing special timestamp filtering logic for lake sources. The changes align the implementation with documented behavior where non-FULL startup modes (EARLIEST, LATEST, TIMESTAMP) read only from log sources, not from lake sources.

Changes:

• Removed test file FlinkUnionReadFromTimestampITCase.java that tested timestamp-based union reads from lake and log
• Simplified startup mode logic in FlinkTableSource.java by removing pushTimeStampFilterToLakeSource method
• Changed TIMESTAMP and EARLIEST modes to not use lake source (only FULL mode now uses lake source when available)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
FlinkUnionReadFromTimestampITCase.java	Deleted test file that tested union reads with timestamp filtering on lake sources
FlinkTableSource.java	Simplified startup mode logic by removing timestamp filter pushdown to lake sources and limiting lake source usage to FULL mode only

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[beryllw]

LGTM！